Prior art systems are known in which the existence of a single resonant circuit in a detection field or zone is utilized as an anti-theft type apparatus. Essentially, if an article having a single resonant frequency tag passes through a detection zone, an alarm is generated which indicates the unauthorized presence of store goods in the detection zone. Such resonant circuits have been constructed in accordance with standard printed circuit board techniques.
Some prior RF tagging systems have provided multiple different tuned (resonant) circuits on a tag so as to specifically identify the goods to which the tag is attached or the destination to which those goods should be directed. Such systems have been proposed for parcel or other article delivery systems wherein resonant circuits are utilized to provide a destination or sender code rather than printed bar codes.
The use of resonant circuit tagging is advantageous in that it is not subject to problems such as dirt obscuring a portion of a printed bar code and causing an error in determining the code associated with the article. Such prior systems utilizing multiple tuned circuit detection contemplate sequentially generating or gating each of the different resonant frequency signals to a transmitter antenna, and then waiting for reflected energy from each of the tuned circuits to be detected. Other frequency tagging systems look for absorption of RF energy by a resonant circuit during the transmission of each test frequency signal.
Prior RF tagging systems contemplate printing the different resonant frequency circuits on a tag in a single plane and then creating different codes by the selective removal of metallization from the resonant circuits. Some systems have recognized that step adjustments of the resonant frequency of such tuned circuits is desirable and this has been implemented by punching holes of predetermined diameters in capacitive elements of the resonant circuit to thereby reduce capacitance and increase the frequency of the resonant circuit.
When it is possible to accurately control the orientation between the resonant multiple frequency tag and the detection zone, some prior systems have noted that fewer different resonant frequencies may be needed to produce the desired end coding result. However, these prior systems accomplish this result by just limiting the number of circuits in the detection zone so that the zone can only accommodate a few different tuned circuits at one time. This has the undesirable effect of effectively requiring wide spacing between tuned circuits on a tag and therefore undesirably increasing the size of the tag on which the tuned circuits are provided.
An improved RF tagging system is fully described in co-pending application Ser. No. 07/966,653, filed on Oct. 26, 1992, in the names of Sanjar Ghaem, Rudyard L. Istvan, and George L. Lauro, for RF Tagging System and RF Tags and Method, which application is assigned to the assignee of the present invention and fully incorporated herein by reference. The system there disclosed includes, as a significant feature, the simultaneous radiation of RF energy at a plurality of different frequencies in order to detect each of a plurality of different frequency resonant circuits which may be provided on a tag. The resonant circuits may be detected by sensing reflected energy from each of the tuned circuits to be detected or by sensing absorption of RF energy by the resonant circuits during the transmission of the simultaneous radiation of RF energy at the plurality of different frequencies. Then a code signal indicative of which resonant frequencies for the tag resonant circuits were detected is provided. The above feature results in fast detection of the resonant frequency circuits provided on a tag. The cross-referenced application further describes an advantageous configuration for step frequency adjusting the resonant frequencies of resonant circuits on a tag and additionally, an RF tagging system which utilizes focused narrow radiation beams for detection of individual resonant circuits on a multiple resonant frequency tag. Also disclosed are preferred RF tag configurations or constructions and a method of making such tags. Additionally, the aforementioned cross-referenced application describes RF tagging system features related to the use of phase shifting or polarization, object approach detection and measuring both voltage and current signals so as to provide improved RF tag detection systems.
In accordance with the present invention, it has been recognized that the prior art technique of forming the resonant circuits in a single plane can greatly limit the number of resonant circuits which may be employed in a given RF tag area. This either limits the number of potential different identifying codes or increases the size of the tags when a greater number of identifying codes is desired. It has also been recognized that by forming the resonant circuits in a single plane, the directional characteristics of the resonant circuits are optimum in a direction normal to the plane of the resonant circuits. This may not always be appropriate for all applications. To that end, it is envisioned that RF tags may be used to advantage in a number of applications other than in applications where the tags are applied to articles. For example, passive RF tags may be used for employee identification purposes or as an implantable transponder. Prior art passive RF tag configurations do not conveniently lend themselves to such applications.
In addition, prior art RF tags have been limited to a particular type of RF tag, either a passive RF tag including only passive resonant circuits which resonate for identification responsive to receiving RF energy or an active RF tag including only a transmitter for transmitting an identifying RF signal. Either type of tag will, in some instances, not respond properly for identification. For example, if some of the passive resonant circuits are shielded from the RF energy, the tag will not be properly identified. For an active RF tag, if the power source to power the transmitter is depleted, or for some reason cannot generate sufficient power to sustain transmitter operation, the active RF tag will not be properly identified. The present invention overcomes these potential problems by providing an RF tag which includes both passive resonant circuits and an RF transmitter.